Apparatus for controlling the speed and phase of engines

ABSTRACT

The specification discloses an arrangement for synchronizing the speeds of and controlling the phase of engines and, in particular, the engines of twin engine airplanes. In the arrangement, one engine is a master engine and the other is a slave engine, and the speeds and phase of the engines are compared and signals are provided in conformity with the results of the comparison to speed up or slow down the slave engine to bring the speed and phase thereof into conformity with that of the master engine if it differs therefrom. In a preferred embodiment of the invention, one of the engines drives a disc having an opaque sector therein and on one side of the disc are photosensitive elements and on the other side are lamps actuated by the other engine in a stroboscopic manner. When the engine speeds and phase match, the opaque sector of the disc prevents the photosensitive elements from being illuminated. The speed of the slave engine is controlled by adjustment of the pitch of a propeller driven thereby and the propeller pitch is, in turn, under the control of a spring loaded engine driven governor. The governor has a further load in the form of a solenoid exerting a magnetic pull thereon and current through the solenoid is under the control of a control system which is sensitive to the illumination of the aforesaid photosensitive elements when the speed of the slave engine, or the phase thereof, departs from that of the master engine.

United States Patent 3,689,175 [451 Sept. 5 1970 Hartzell et al.

[54] APPARATUS FOR CONTROLLING THE SPEED AND PHASE OF ENGINES [72] Inventors: James R. Harwell, Troy; Arley J.

Reas, Piqua, both of Ohio [73] Assignee: Piqua Aircraft Co. (Inc.), Piqua,

Ohio

[22] Filed: Aug. 11, 1970 [21] App1.No.: 62,859

[52] US. Cl. ..416/34, 60/975, 416/35 [51] Int. Cl ..B64c 11/50 [58] Field of Search ..416/34, 35; 60/975; 91/458;

[56] References Cited UNITED STATES PATENTS 2,477,012 7/ 1949 Sorensen ..60/97 S 2,797,913 7/1957 Moulton ..73/518 3,066,741 12/1962 Barnes ..416/34 3,458,709 7/1969 Beall et a1 ..340/271 FOREIGN PATENTS OR APPLICATIONS 1,054,288 4/1959 Switzerland ..192/84 Primary ExaminerMartin P. Schwadron Assistant Examiner-Clemens Schlmlkowski Attomey-Melvin A. Crosby ABSTRACT The specification discloses an arrangement for synchronizing the speeds of and controlling the phase of engines and, in particular, the engines of twin engine airplanes. In the arrangement, one engine is a master engine and the other is a slave engine, and the speeds and phase of the engines are compared and signals are provided in conformity with the results of the comparison to speed up or slow down the slave engine to bring the speed and phase thereof into conformity with that of the master engine if it differs therefrom.

In a preferred embodiment of the invention, one of the engines drives a disc having an opaque sector therein and on one side of the disc are photosensitive elements and on the other side are lamps actuated by the other engine in a stroboscopic manner. When the engine speeds and phase match, the opaque sector of the disc prevents the photosensitive elements from being illuminated. The speed of the slave engine is controlled by adjustment of the pitch of a propeller driven thereby and the propeller pitch is, in turn, under the control of a spring loaded engine driven governor. The governor has a further load in the form of a solenoid exerting a magnetic pull thereon and current through the solenoid is under the control of a control system which is sensitive to the illumination of the aforesaid photosensitive elements when the speed of the slave engine, or the phase thereof, departs from that of the master engine.

28 Claims, 7 Drawing Figures l 1 l3 MASTER ENGlNE l2 a 5LAVE ENCalNE PULS E m nrn 2G CONTROL voren'riomsren 501.1!010 CONTROL I senvo m sure Rea. M 5

FUNCTION SWlTCH P'A'IENTEDSEP 51912 3.689.175

SHEET 1 OF 4 T INVENTORS JAMES R. HARTZELL Y ARLEY J REAs FIG-2 INVENTORS .JRMES R. HARTZELL BY ARLEY J.REAS

PATENTEDsEP 5 I972 SHEET 3 BF 4 INVENTORS JAME5 R. HARTZELL R Y J. REAS SHEET 0F 4 INVENTORS JAMES N. HARTZELL ARLEY J. REAs b PNENTEDsEP 51212 APPARATUS FOR CONTROLLING THE SPEED AND PHASE OF ENGINES The present invention relates to an apparatus for controlling the speed and phase of engines and, in particular, for controlling the speed and phase of a slave engine relative to that of a master engine in an aircraft.

Many systems have been devised for synchronizing and phasing aircraft engines but these have, in general, been relatively complex and, furthermore, have generally been provided only for larger aircraft having four or more engines.

The control of the speed and phase of aircraft engines is highly important because undesirable vibrations and noises can be set up when engines get out of a predetermined phase relationship or when the engines run at different speeds. Such vibrations are not only objectionable but can even be dangerous because of the magnitude of the vibration that can be established in the aircraft thereby.

A common type aircraft engine utilizes a variable pitch propeller and the spe ed of the engine connected to the propeller is controlled by varying the pitch of the propeller so that substantially constant speed engine conditions can be maintained. The pitch of the propeller is controlled by a governor driven by the respective engine so that changes of the speed of the engine adjust the governor and thus adjust the pitch of the propeller, usually by way of a hydraulic medium, and thereby the speed of the engine is maintained sub stantially constant. The adjustment of such governors are usually accomplished by adjusting the spring bias thereon which will control the response of the governor at a given engine speed.

Having the foregoing in mind, a primary objective of the present invention is the provision of an improved and simplified system for automatically and continuously maintaining the speed and phase of a pair of engines in a predetermined relation to each other.

Another object of this invention is the provision of a method and apparatus for causing the speed and phase of a slave engine to conform to that of a master engine.

Still another object of this invention is the provision of a novel arrangement for use in a system of the nature referred to above for controlling the response of the governor of the slave engine at any given speed thereof.

A still further object of the present invention is the provision of a method and apparatus for controlling the speed and phase of a slave engine in an aircraft having at least two engines, relative to that of the master engine of the aircraft and which can readily be adapted to existing aircraft as well as forming an installation that can be incorporated in new manufacture.

A particular object of the present invention is the provision of an arrangement for adjusting the response of a governor which is driven by an engine at any given engine speed by electromagnetic means.

The foregoing objections and advantages of the present invention, as well as still other objects and advantages thereof will become more apparent upon reference to the following detailed specification, taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic view of a twin engine aircraft showing the location therein of the important components of the present invention;

FIG. 2 is a sectional view showing a typical governor employed for controlling the speed of an engine;

FIG. 3 is a schematic representation of the speed comparing arrangement that is employed for developing speed and phase correcting signals; 1

FIG. 4 is a schematic view showing a pulse generating arrangement for being driven by one of the engines to develop pulses for use in the comparing arrangement of FIG. 3;

FIG. 5 is a schematic representation of the pulse amplifying circuit to which the pulse generating device of FIG. 4 is connected;

FIG. 6 is a schematic representation of the control circuit which is actuated by the comparing means for developing speed correcting signals for the slave engine; and

FIG. 7 shows the control circuit of the present invention in the form of a block diagram.

SUMMARY OF THE INVENTION According to the present invention a master engine is provided, the 'speed of which is under the control of a manual throttle or the like. A slave engine is provided which is brought into conformity as to speed and phase with that of the master engine. Each engine drives a propeller which is variable in pitch and the pitch of the propellers is under the control of respective engine driven governors. Each propeller is of a known type in which the pitch is adjusted by the control of a supply of hydraulic fluid thereto. Each governor includes a valve for regulating the flow, of hydraulic fluid to and from the respective propeller pitch changing mechanism.

According to the present invention, the governor driven by the slave engine is provided with an electromagnetic arrangement for adjusting the response of the governor at a given engine speed. Speed comparing means are provided and can take the form of a disc driven by one of the engines and having an opaque sector therein. On one side of the disc there is mounted photosensitive elements and on the other side of the disc is mounted stroboscopic lights which are actuated slave engine is in conformity with that of the master engine the opaque sector of the disc blanks off illumination from the lamps to the light sensitive elements. When, however, the slave engine either leads or trails the master engine, one or the other of the photosensitive elements becomes illuminated and this supplies a signal to the control circuit that adjusts the supply of energy to the electromagnetic means that is controlling the response of the governor associated with the slave engine and in a direction to restore the slave engine to the proper speed and phase and which will bring the opaque sector of the disc back into position to exclude light from the photosensitive elements.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings somewhat more in detail, FIG. 1 shows an aircraft having engines 10 and 12 driving respective variable pitch propellers 14 and 16 of the type in which the pitch is regulated by a supply of hydraulic fluid.

One of the engines drives a pulse generator indicated at 18 and the other of the engines has a take-off at 20 for driving a disc forming a part of a speed and phase comparing device.

The control circuit under the control of the comparing device is indicated at 22 and may be mounted in the main fuselage portion of the aircraft in a position accessible to the crew of the aircraft. Also mounted in position to be available to the pilot is a cycle switch 24 to be described hereinafter.

Either of the engines or l2 can be the master engine and the other thereof can be the slave engine and, purely for the purpose of illustration, engine 12 is selected as the slave engine. Associated with slave engine 12 is an engine driven governor 26 for controlling the supply of fluid to the propeller pitch adjusting mechanism.

In operation,'the speed of engine 10 is set, as by a manual throttle, and a governor associated with the engine controls the supply of fluid to the propeller pitch adjusting mechanism so as to maintain the engine speed substantially constant. The governor 26 for slave engine 12 is arranged to maintain the speed of engine 12 at about that of engine 10, but when engine 12 leads or trails engine 10 the speed comparing arrangement of the present invention detects the condition and supplies a signal to an. electromagnetic device associated with governor 26 to adjust the governor response in such a direction as to bring the speed and phase of engine 12 back into conformity with that of engine 10.

Turning now to FIG. 2, there is illustrated therein the governor 26 for the slave engine, it being understood that a similar governor without the electromagnetic control device could be employed for the master engine as well.

Governor 26 comprises a valve body 30 adapted for being stationarily supported while a shaft 32 is adapted for being driven by the respective engine and will cause a carrier 34 in the valve to rotate. Carrier 34 carries the flyballs 36 which work against the load of a spring 38 in a conventional manner. The position of the flyballs controls the position of a valve spool that regulates the supply of pressure fluid to and the exhaust of pressure fluid from the propeller adjusting device in a conventional manner.

If the propeller is of the type in which the pitch reduces in response to an increase in volume of oil and increases in'response to a decrease in the volume of oil then, as flyballs 36 move outwardly from the predetermined position, indicating overspeed of the engine, the valve is shifted in a manner to release fluid from the pitch adjusting mechanism, thereby to permit the propeller pitch to increase and to cause the engine connected thereto to slow down.

If, on the other hand, the propeller is of the type in which the pitch increases with an increase in volume of oil and decreases with a decrease in the volume of oil then, when the flyballs 36 move outwardly from a predetermined position the supply of pressure fluid to the propeller pitch adjusting mechanism is increased and the pitch of the propeller increases and causes the engine to slow down. Either of the aforementioned known systems is contemplated within the preview of the present invention.

The particular feature about the governor arrangement of FIG. 2, and which is substantially conventional as to the basic components thereof, is the provision of a solenoid 40 pertaining thereto which is operatively as sociated with a plunger 42 of magnetic material connected to the spool of the valve. It will be seen that energization of coil 40 will cause a magnetic force to act on plunger 42 so as to bias the spool of the valve along the same line of action as it is biased by spring 38. As will be seen hereinafter in normal operation, coil 40 is supplied with a current flow of a predetermined magnitude and this supplies a certain bias to the valve spool in opposition to the bias exerted by spring 38. By increasing the current to coil 40 the bias can be increased and by decreasing the current to coil 40 the bias can be decreased. It will be apparent that the response of the governor to any given engine speed can thus be readily controlled by controlling the current supply to coil 40.

The coil 40 in FIG. 2 is shown as exerting an upward pull on core or plunger 42 but it is obvious that the force magnetically exerted on the plunger or core 42 by coil 40 could be in the downward direction. Further, it is conceivable for the coil to be associated with the flyballs of the governor and to influence the movement thereof by exerting more or less magnetic bias thereon in the direction of movement thereof. Still further, it is conceivable for the coil to be arranged around the valve and to exert a bias directly on plunger 40. In this case, the valve body would be of non-magnetic material, such as aluminum, while the valve spool would be formed of magnetic material, such as steel. In any case, it will be evident that the response of the governor at any given engine speed is under the control of the electromagnetic arrangement and the electromagnetic arrangement can be caused to adjust the response of the governor in either direction.

FIG. 3 schematically illustrates the speed comparing arrangement of the present invention. In FIG. 3, a disc is mounted on a shaft 52 and is driven in rotation by one of the engines, in this case, engine 12. The disc comprises transparent material and has an opaque sector at 54. On one side of the disc is stationarily mounted a support member 56 on which is disposed a pair of photosensitive elements 58 and 60 which are spaced apart a distance less than the circumferential spacing of the limits of opaque sector 54 so that opaque section 54 in a certain position thereof will cover the photosensitive elements. When the opaque sector moves in either direction from the said position, one or the other of the elements will be exposed for illumination by the respective lamp.

For illuminating the elements there is mounted on the side of the disc 50 opposite support member 56 another support member 62 having a stroboscopic, or flash, lamps 64 and 66 therein. These flash lamps are actuated by the engine which is not connected to disc 50, in this case, engine 10. Actuation of the flash lamps 64 and 66 is accomplished by the pulse generator 18, previously referred to, which develops pulses as it is rotated by engine 10.

When the engines are in proper speed and phase relation, lamps 64 and 66 flash when disc 50 has the opaque sector 54 thereof disposed in covering relation to photosensitive elements 58 and 60. As mentioned,

however, when the sector 54 leads or trails the said position, one or the other of photosensitive elements 58 and 60 is exposed to the illumination from the respective stroboscopic lamp 64, 66 and is illuminated thereby when the lamp flashes so as to provide a control pulse. While two light sources are illustrated, it will be understood that one light source could be employed because both of the lamps flash at the same instant. Also, either engine could actuate the disc and the other of the lamps.

Preferably, at least one of the disc 50 and the photosensitive elements 58, 60 together with lamps 64, 66 is angularly adjustable about the axis of shaft 52 relative to the other.

FIG. 4 schematically illustrates the pulse generator associated with engine 10. In FIG. 4, it will be seen that the pulse generator comprises a magnetic frame 70 with a magnetic rotor 72 therein mounted on a shaft 74 drivingly connected with the respective engine. Rotor 72 carries a permanent magnet 76 which moves past one end of a coil 78 having a magnetic core 80 therein once for each complete revolution of rotor 72. The peripheral extent of magnet 76 is relatively short and, as a result, a relatively sharp strong pulse of voltage is induced in coil 78 when magnet 76 moves past the coil which serves to control the stroboscopic light in the form of flash lamps 64 and 66 of FIG. 3.

FIG. 5 shows the pulse amplifying circuitry that provides for illumination of flash lamps 64 and 66 in response to pulses generated in coil 78 by movement of magnet 76 past the said coil.

In FIG. 4, a pulse of voltage of the proper polarity in coil 78 will cause current to flow through a resistor R1 which has an adjustable tap 90 connected to the base of an NPN transistor T1, the emitter of which is connected to ground and the collector of which is connected to the base of the PNP transistor T2. When current flows through resistor R1 to ground, transistor T1 will go conductive and the base of transistor T2 will go toward ground. The emitter of transistor T2 is connected through a resistor R2 and a choke coil C1 to a source of plus voltage at 92 so that when the base of transistor T2 goes toward ground, transistor T2 will conduct and supply a plus voltage via the collector thereof to the base of an NPN transistor T3, the emitter of which is connected to ground via a resistor R3 and the collector of which is connected to one end of the primary coil of a transformer 93, the other end of the said primary being connected back through choke C1 to the plus voltage source at 92. A capacitor 94 is preferably connected between the said other end of the primary transformer 93 and ground.

The secondary of transformer 93 has one end connected to ground and a capacitor 96 is connected across the terminals of the secondary. The end of the secondary winding of transformer 93 which is not connected to ground is connected to one end of each of three resistors R4, R5 and R6, the other ends of which are connected to respective sides of the aforementioned flash lamps 64 and 66 and still another flash lamp 98. The other sides of the flash lamps are connected to ground. It will be apparent that a single pulse developed in coil 78 is amplified to supply a strong charging pulse to capacitor 96 and, when the voltage on the capacitor reaches the voltage at which lamps 64 and 66 and 98 will conduct, the lamps will conduct and drain off the charge on capacitor 96, producing a single strong flash of light in the process. Lamp 98 is not used in the system except as a signal light to show that the system is operating.

Preferably, at least one of the magnet 72 and coil 78 v is angularly adjustable relative to the other for phase adjustment.

The manner in which signals developed through illumination of photosensitive elements 58 and 60 are caused to adjust the current to solenoid coil 40 in order to adjust the response of governor 26 to a given speed of the pertaining engine as shown in FIG. 6. In FIG. 6, it will be seen that coil 40, associated with the governor, has one side connected to a source of plus voltage at 100, while the other side is connected to the collector of an NPN transistor T4, the emitter of which is connected through a resistor R7 to ground. The source at 100 is also connected to ground through the serially connected resistors R8, R9 and R10 with the juncture of resistors R8 and R9 connected to the base of transistor T4. Resistor R9 is in the form of a rheostat and it will be apparent that adjustment thereof will adjust the bias to the base of transistor T4 and modify the rate of current flow through solenoid coil 40.

Rheostat R9 is operatively connected to an adjusting motor M which is reversibly energizable to adjust rheostat R9 in respectively opposite directions, thereby to adjust the current flow in solenoid coil 40 in respectively opposite directions.

A second source of positive voltage, preferably an extension of source 100, is indicated at 102 and this point is connected through a relay blade 104 with the common juncture between photosensitive elements 58 and 60. These elements when dark have high resistance and no current flow takes place therethrough. However, upon illuminating one or the other of the photosensitive elements; for example, photosensitive element 58, current flow will take place therethrough and through resistors R11 and R12 to ground. The positive voltage thereby established at the juncture of resistors R11 and R12 is conveyed through resistor R13 to the base of an NPN transistor T5, the emitter of which is connected through resistor R14 to ground and also to the emitter of another NPN transistor T16. The collector of transistor T5 is connected through a resistor R15 with the plus source at 100 and also through resistor R16 with the base of a PNP transistor T6. When transistor T5 goes to conduction, a positive bias will be supplied to the base of transistor T6 and, since the emitter thereof is connected by resistor R17 with the plus source at 100, transistor T6 will also go to conduction. The collector of transistor T6 is connected by a resistor R18 to ground and also with the base of an NPN transistor T7. The collector of transistor T7 is connected to source 100 and the emitter thereof is connected to the base of a still further NPN transistor T8, the collector of which is connected to plus source 100 and the emitter of which is connected with one side of motor M at 104.

The emitter of transistor T7 is also connected through resistor R19 with the base of a still further NPN transistor T9, the emitter of which is grounded and the collector of which is connected to the opposite side of motor M at 106. It will be evident that when transistor T7 goes to conduction, a positive bias will be supplied to the bases of both of transistors T8 and T9, and these transistors will, therefore, go to conduction and cause current flow through motor M in the direction from 104 to 106 and cause the motor M to rotate in one direction to adjust rheostat R9 and thereby adjust the bias of the base of transistor T4 and change the rate of current flow in coil 40.

Simultaneously with the development of a signal at the juncture of resistors R11 and R12, the said signal is supplied via a resistor R20 to the base of an NPN transistor T10, the emitter of which is connected to ground via a resistor R21, while the collector thereof is connected to the juncture of resistors R22 and R23 which, with respect to light sensitive element '62, correspond to resistors R1 1 and R12 for light sensitive element 58. Inasmuch as transistor T10 goes to conduction upon the development of a signal due to illumination of light sensitive development 58 and resistor R21 is selected so as to have a relatively small value, no signal can be developed at the base of transistor T6, but instead, any such signal that might accidentally be developed will be grounded out through transistor T10 and resistor R21.

Similarly, if light sensitive element 60 is illuminated and commences to conduct so that current will flow through its resistors R22 and R23 to ground, the positive signal developed at the juncture of resistors R22 and R23 will be conveyed byresistor R24 to the base of NPN transistor T11, the collector of which is connected to the juncture of resistors R11 and R12 and the emitter of which is connected with the emitter of transistor T10 and to the ungrounded end of resistor R21. It will be evident that whenever light sensitive element 60 is illuminated, any signal that might be supplied to the base of transistor T5 by accident is grounded out through transistor T11 and resistor R21. In this manner the development of a signal at either of the photosensitive elements locks out any signal from the other thereof.

'The circuitry pertaining to light sensitive element 60 is identical with that pertaining to light sensitive element 58 and is not described in specific detail except to note that NPN transistor Tl6for light sensitive element 60 corresponds to transistor T5 for light sensitive element 58. Furthermore, PNP transistor T12 for light sensitive element 60 corresponds to PNP transistor T6 pertaining to light sensitive element 58. Also, NPN transistor T13 following transistor T12 corresponds to transistor T7 following transistor T6 and NPN transistors T14 and T15, following transistor T13, correspond to transistors T8 and T9.

When, however,'light sensitive element 60 is illuminated and causes transistors T14 and T15 to go to conduction it will be noted that the current flow from source 100 is through transistor T14 and then to point 106 and through motor M toward point 104 and then through transistor T15 to ground. This causes rotation of motor M in a direction opposite to that which obtains when light sensitive element 58 is illuminated and effects the adjustment of rheostat R9 in the opposite direction.

It will be evident from the foregoing that the illumination of either one of the light sensitive elements 58 and 60 will bring about'energization of motor M in a direction to adjust rheostat R9 to adjust the current flow through coil 40 in a direction to modify the response of governor 26 in a direction to correct for the difierence in speed or phase of the slave engine which initially caused the illumination of the respective photosensitive elements.

It is also desirable to provide for extremely rapid adjustment of the current through coil 40 and for adjustmentthereof in response to small or transient signals.

The circuit of FIG. 6 includes components for effecting the above mentioned control. For this purpose, source is connected to thecollector of an NPN transistor T17, the emitter of which is connected through the resistor R25, with the collector of another NPN transistor T18, the emitter of which is connected through resistor R26 to ground. The juncture of the collector of transistor T18 and resistor R25 is connected with the base of transistor T4.

The base of transistor T17 is connected by a resistor R27 with the slider on adjustable resistor R28 which has one end connected to ground and the other end connected to terminal 106 pertaining to switch blade 108 of the relay which also includes the aforementioned switch blade 104. Blade 108 is, in turn, connected with the base of transistor T7. It will be apparent that positive signals supplied to the base of transistor T7 will also be supplied via resistor R28 and resistor R27 to the base of transistor T17 and modify the conductivity thereof so as to produce instantaneous adjustment of the bias standing on the base of transistor T4. Thus, before motor M can operate, or in response to a transient signal, or the like, developed by light sensitive element 58, there is already an adjustment of the bias on the base of transistor T4.

Similarly, the base of transistor T18 is connected by a resistor R29 with a slider adjustable along resistor R30 which has one end grounded and another end connected to a terminal 110 pertaining to a blade 112 of the aforementioned relay and which blade is connected to the base of transistor T13. By this arrangement, signals developed by illumination of light sensitive'element 60 will be substantially instantaneously transmitted to the base of transistor T18 to effect an adjustment in the bias at the base of transistor T4.

A still. further control is provided in the form of a centering system. In normal operation, coil 40 draws current continuously at about, say, half the maximum permissible amount and the rate of current flowing therethrough is adjusted in opposite directions from the intermediate amount in order to adjust the response of the governor associated therewith. Occasionally, it may occur that the amount of current through coil 40 differs considerable from the aforementioned intermediate amount and the circuit of FIG. 6 provides a centering arrangement for restoring the current in coil 40 to about the aforementioned intermediate amount.

This system comprises the aforementioned centering switch 24 which, as will be seen, will connect the coil 114 of the relay controlling blades coil 104, 108 and 112 between the plus voltage source 102 and ground and thereby energize the coil and move the blades downwardly. When blades 108 and 112 are moved downwardly they merely separate from their respective contacts 106 and 110 and thereby isolate the bases of transistors T17 and T18 from any source of bias voltage.

Blade 104, which supplies voltage to photosensitive elements 58 and 60, separates from its contact and, instead, engages a contact which is connected to one end of the serially arranged resistors R31 and R32 which have the other end connected to ground. The juncture of the resistors R31 and R32 is connected, on the one hand, by a resistor R33 with the emitter of an NPN transistor T19 and, on the other hand, through resistor R34 with the base of an NPN transistor T20. The juncture of the collector of transistor T4 and coil 40 is connected by a resistor R35 with the base of transistor T19 and by resistor R36 with the emitter of transistor T20.

The collector of transistor T19 is connected with the base of transistor T6, while the collector of transistor T19 is connected with the base of transistor T12.

It will be apparent that when switch 24 is closed to energize relay coil 1 l4, resistors R31 and R32 form two legs of a bridge, the other two legs of which consist of coil 40, and the collector emitter path of transistor T4 together with resistor R37. One end of each pair of legs is grounded and the other end thereof is connected to the positive voltage source and the intermediate points, represented by the juncture of coil 40 and the collector of transistors T4 and the juncture of resistors R31 and R32 are connected to the bases of transistors T19 and T20, respectively.

If, at the time of actuating switch 24 the current through coil 40 departs substantially from the aforementioned intermediate amount, one or the other of transistors T19 and T20 will be biased to conduction and the other will be biased to cut-off. The one of the said transistors which goes conductive will develop a negative bias at the base of the respective one of transistors T6, and T12, which will lead to energization of motor M to adjust resistor R9 in a direction to restore the balance across the bridge, whereupon the conductive one of transistors T19 and T20 will cease conducting and motor M will stop running. In this manner, the current through coil 40 can readily be restored to the desirable intermediate value which leaves the maximum amount on either side of the said intermediate value for control purposes. 1

Upon releasing centering switch 24, the system is restored to its original operating condition and transistors T19 and T20 go to idle condition and are not functional until switch 24 is again closed.

FIG. 7 schematically illustrates the control system of the present invention in the form of a block diagram. Certain components in FIG. 7 carry the reference numerals previously employed and others thereof have legends applied.

In FIG. 7, it will be seen that master engine 10 has a governor 11 and a control throttle 13. As mentioned previously, the adjustable pitch propeller driven by engine 10 is adjusted in pitch by governor 11 to maintain engine 10 at substantially constant speed. The master engine drives pulse generator 18 and the pulses therefrom are amplified by the pulse amplifier, identified by a legend, and consisting of the amplifying components of FIG. 5.

The output from the pulse amplifier is supplied to the stroboscopic sensing unit 22 identified by a legend and consisting of the lamps and photosensitive elements and rotating disc of FIG. 3 or the functional equivalants thereof. One element of the sensor 22 is supplied by the signals from pulse generator 18, which have been amplified in the pulse amplifier, and another element is driven by slave engine 12. Slave engine 12 has a governor 26 associated therewith for controlling the speed of the slave engine and associated with this governor is the electromagnetic means consisting of coil means 40 and armature means 42.

As previously described, the electromagnetic means supplied a variable bias to governor 26 depending upon the degree of energization of coil means 40.

The signals from the stroboscopic sensing unit 22 are supplied through respective amplifiers to respective servo amplifiers which are connected for reversibly energizing servo motor M. Servo motor M is preferably connected through a gear reducer S with the slider of the control potentiometer shown at R9 in FIG. 6. The control potentiometer is connected to a solenoid control unit which controls the amount of energy supplied to coil means 40.

The servo amplifiers are in circuit with the function switch of FIG. 6' and connected thereto is the previously described control switch 24.

FIG. 7 will serve to show how the several components of the control system are interconnected to effect the control functions which have been described previously.

From the foregoing it will be appreciated that the arrangement of the present invention not only synchronizes the engines propellers but is also operable for phasing the propellers at the same time.

Either of the actuator for the light sources, or the rotating disc, or the position of the flash lamps and photosensitive elements relative to the disc could be adjusted in order to obtain the desired phase angle of the engines and propellers'as might be desired.

It is suggested that neon type bulbs can be employed as the light source but other types of discharge bulbs and circuitry therefor can be employed. Further, the photosensitive elements could be photosensitive resistors, but it is also possible to employ phototransistors, photo-diodes or photovoltaic cells, as might be desired.

It has been mentioned that the solenoid coil for adjusting the response to the governor could operate on the governor spool or on a magnetic core attached thereto, but it could also operate to provide a variable magnetic pull to the flyballs themselves.

The system is sensitive as to the direction of rotation and when sensing either an overspeed or an underspeed of the slave engine, the control in respect of the detection of speed maladjustment in the opposite direction is completely locked out until the engine is first restored to the proper speed and phase relation.

By using two separate controls for controlling the current supply to the governor coil it becomes possible for the two systems to work on opposite sides of a fixed value of current to the coil. The system of the present invention, as mentioned, can be incorporated in new manufacture but, as will be appreciated, it is also readily adaptable for incorporation in existing twin engine aircraft.

Modifications may be made within the purview of the appended claims.

WHAT IS CLAIMED IS:

1. In combination: a master engine, a slave engine, a variable pitch propeller having pitch adjusting means and driven by said slave engine, a spring loaded governor driven by said slave engine and controlling said pitch adjusting means and operable to cause the propeller pitch to vary directly with variations in speed of said slave engine, adjustable biasing means operatively associated with said governor and energizable for adjusting the load on said governor to adjust the photosensitive means response of said governor to engine speed, means establishing a speed standard comprising an element actuated. by said master engine, comparing means for comparing the speed of said slave engine with said speed standard and operable to develop a signal in response to a difference between said speeds, a source of energy for said biasing means,- and control means operable in response to said signal for controlling the supply of energy from said source to said biasing means to adjust the bias on said governor and in a direction to reduce said difference, said comparing means comprises photosensitive means and lamp means for illuminating said photosensitive means and a rotary member between said lamp means and said photosensitive means having one rotated position wherein the light from said lamp means is blocked off from said photosensitive means, said element driven by said master engine being operable for actuating one of said rotary member and lamp means while said slave engine actuates the other thereof, and illumination of said providing said signal for actuating said control means. I 2. The combination according to claim 1 in which said rotary member is a disc having an opaque portion which is disposed between said lamp means and photosensitive means in said one rotated position of said disc, said lamp means when actuated being momentarily energized in a predetermined rotated position of the actuating engine therefor to provide stroboscopic light source means.

3. The combination according to claim 2 in which said photosensitive means comprises a pair of photosensitive elements each operable to provide a signal for actuation of said control means for adjustment of the supply of energy from said source to said biasing means in a respective direction when illuminated, said disc in said one rotated position thereof blocking off the light from said lamp means to both of said photosensitive elements and when displaced in respective directions from said one rotated position exposing respective ones of said photosensitive elements to the light from said lamp means. 4. The combination according-to claim 3 in which said biasing means is electrically operable and they said source includes a source of electrical energy, said control means comprising first and second control circuits each operable when completed to provide for adjustment of the supply of energy from said source to said biasing means in a respective direction, each photosensitive element being connected in controlling relation to a respective one of said circuits.

5. The combination according to claim 4 which includes means responsive to the completion of either of said first and second control circuits for preventing the completion of the other thereof.

6. The combination according to claim 4 in which said biasing means is in the form of a coil operable when energized to establish a magnetic field, said governor including centrifugally operable means comprising at least one magnetic part and movable in response to changes in engine speed, said coil being so mounted as to cause the field thereupon to act on said magnetic part of said governor and in the direction of movement thereof whereby the total load on the governor which controls the governor response to engine speed includes the spring load thereon and the magnetic load thereon, a change in the supply of energy to said coil changing the position of the governor for a given speed of the slave engine.

7. The combination according to claim 6 in which said governor comprises a reciprocable valve member and said magnetic part is integral therewith.

8. The combination according to claim 6 in which said governor comprises flyball members, said flyball members being magnetic and forming the magnetic motor being connected in adjusting relation to said,

rheostat.

11. The combination according to claim 1 in which said biasing means is a solenoid coil and said source of energy comprises a source of direct current, a current controlling transistor having the collector-emitter path thereof in series with said coil across said source, biasing resistor means connected across said source and to the base of current controlling transistor, said control means including an adjustable element associated with said resistor means movable for varying the conductivity of said current controlling transistor, a reversible motor connected to said element for adjustment thereof, and energizing circuit means connected to said motor and under the control of said signals from said comparing means.

12. The combination according to claim 11 in which said energizing circuit means includes first and second transistorized circuits connected to said motor and each effective for causing rotation of the movement of the motor in a respective direction, said first circuit becoming effective in response to a said signal from said comparing means resulting from a leading condition of said slave motor and said second circuit becoming effective in response to a said signal resulting from a trailing condition of said slave motor, each of said first and second circuits having a control terminal to receive the respective signal from said comparing means.

13. The combination according to claim 12 which includes a transistor for each control terminal having the collector-emitter path thereof connected between ground and the respective control terminal, each transistor having the base thereof connected to the other control terminal whereby a signal supplied to one control terminal will prevent the supply of a signal to the other control terminal.

14. The combination according to claim 12, in which said resistor means connected to the base of said current controlling means comprises a pair of resistors connected in series across said source and having the juncture thereof connected to the base of said current controlling transistor, a further transistor having the collector-emitter path thereof connected across each of said pair of resistors, and the base of each further resistor being connected to a point in a respective one of said first and second circuits to receive a biasing signal therefrom when the respective circuit is actuated by a signal from said comparing means.

15. The combination according to claim 13, which includes a second pair of resistors connected in series, switch means selectively operable for interrupting the connection between the bases of each of said further transistors and the respective said first and second circuits while simultaneously connecting said second pairs of resistors across said source, and means connected between the juncture of said coil with the collectoremitter path of said current controlling transistor and the juncture of said second pair of resistors and responsive to the voltage between said junctures to make the one of said first and second circuits effective which will lead to adjustment of said element in a direction to reduce said voltage.

16. The combination according to claim 15 in which said last mentioned means comprises a pair of transistors each having the emitter connected to arespective one of said junctures and the base connected to the other thereof whereby each of the transistors goes conductive in response to a voltage between said junctures of a respective polarity, the collectors of the said transistors being connected to said first and second circuits to supply actuating signals thereto when the respective transistor goes conductive.

17. The combination according to claim 1 in which said means establishing a speed standard also includes means for establishing a phase standard, said comparing means also comparing the phase of said slave engine with that of said master engine and developing a said signal in conformity with measured differences in phase.

18. In a twin-engine aircraft having a master engine and a slave engine and a constant speed propeller driven by each engine and each propeller including a fluid motor to adjust the pitch thereof; means for synchronizing the speed of said engines, comprising: a speed sensing fluid governor driven by each said engine, each said governor incorporating a pilot valve regulating the flow of fluid to the respective said fluid motor for maintaining the respective propeller at the proper pitch for a prescribed rotational speed of the engine connected thereto, each said pilot valve being resiliently biased toward pitch reducing position and each governor urging the respective pilot valve toward pitch increasing position as the speed of the pertaining engine increases, said governor driven by said slave engine incorporating unidirectionally acting magnetic means operatively associated with said pilot valve and adapted to develop a unidirectional bias thereon, said magnetic means comprising magnetic coil means and magnet means unidirectionally influenced by the field of said magnetic coil means, one of said magnet means and magnet coil means being connected to thesaid pilot valve to move therewith and the other thereof being stationarily mounted, an energizing circuit connected to said magnetic coil means to supply energy thereto and including a source of electrical energy, speed comparing means comprising an element actuated by each engine and operable for comparing the rotational speed of said master engine with that of said slave engine, said speed comparing means developing signals which vary in conformity with the relative rotational speed of said engines, and means operatively associated with said energizing circuit and connected to receive said signals and operable for varying the degree of energization of said magnetic coil means by said energizing circuit inresponse to variations in said signals, variations in the degree of energization of said magnetic coil means adjusting the bias exerted by said magnetic means on said pilot valve of the slave engine governor and in a direction to tend to bring said speed of said slave engine into synchronism with that of said master engine;

19. The combination according to claim 18 in which said comparing means comprises a rotating magnet driven by said master engine and a stationary coil adjacent to and cooperating with said magnet to emit electrical pulses in unison with the movement of said magnet pastsaid coil, amplifying means to amplify said electricalpulses, said comparing means also comprising a rotatable shaft driven by said slave engine, a pair of fixed photosensitive elements angularly spaced less than about said shaft, a pair of fixed lamps facing said photosensitive elements, means for momentarily illuminating said lamps by the amplified electrical pulses from said amplifying means, a disc located between said lamps and said photosensitive means and mounted for rotation with said shaft, said disc being opaque over a sufficient portion of its surface to block off the light from both lamps to said photosensitive elements when in the null position, said disc exposing one photosensitive element to a light pulse when angularly displaced slightly in one direction from said null position and exposing the other photosensitive element to a light pulse when angularly displaced slightly in the other direction from said null position, each photosensitive element emitting an electrical signal when illuminated, means for amplifying said electrical signals, one said photosensitive elements emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in one direction, the other said photosensitive element emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in the opposite direction, thereby tending to bring the speed of said slave engine into synchronization with said master engine when said opaque portion of said disc is out of the null position at the instant when the light pulses occur.

20. The combination according to claim 19 wherein said rotating magnet is driven by said slave engine and said rotating shaft is driven by said master engine.

21. The combination according to claim 19, including means for adjusting the angular position of said photosensitive elements, together with the corresponding lamps, about said rotatable shaft.

22. The combination according to claim 19, including means for adjusting the angular position of said coil about the shaft of said rotating magnet.

23. The combination according to claim 19, including means for automatically controlling the current supplied to said magnetic coils according to a prescribed schedule when said disc is not in the null position.

24. In combination according to claim 19, including means for energizing said magnetic coils with a constant current in one direction to provide a reference bias on said pilot valve, said means including means for automatically changing the degree of bias when said disc is displaced from the null position a predetermined amount. 1

25. In combination according to claim 24 in which said means for automatically changing the degree of bias includes a motor driven potentiometer for the pur-- pose of changing the current supplied to said magnetic coil.

26. In combination according to claim 19 including means for blocking out the signal emitted by the second photosensitiveelement which becomes uncovered by the opaque portion of said disc after the said opaque portion becomes displaced from said null position, in order that the signal from the first photosensitive element which becomes uncovered can act without opposition from said second photosensitive element to energize said coil in a directionto correct the out of synchronous condition when the rotational speed of said slave engine is above or below the speed of said master'engine by a prescribed amount.

27. In a twin-engine aircraft having a master engine and a slave engine and a constant speed propeller driven by each engine and each propeller including a fluid motor to adjust the pitch thereof; means for synchronizing the speed of said engines, comprising: a speed sensing fluid governor driven by each said engine, each said governor incorporating a pilot valve regulating the flow of fluid to the respective said fluid motor for maintaining the respective propeller at the proper pitch for a prescribed rotational speed of the engine connected thereto, each said pilot valve being resiliently biased toward pitch reducing position and each governor urging the respective pilot valve toward pitch increasing position as the speed of the pertaining engine increases, said governor driven by said slave engine incorporating unidirectionally acting magnetic means operatively associated with said pilot valve and adapted to develop a unidirectional bias thereon, said magnetic means comprising magnetic coil means and magnet means unidirectionally influenced by the field of said magnetic coil means, one of said magnet means and magnet coil means being connected to the said pilot valve to move therewith and the other thereof being stationarily mounted, an energizing circuit connected to said magnetic coil means to supply energy thereto and including a source of electrical energy, comparing means comprising an element actuated by each engine and operable for comparing the angular position of the propeller driven by said slave engine with the angular position of the propeller driven by said master engine, said comparing means developing first signals when said propeller driven by the slave engine leads a predetermined position relative to the propeller driven by the master engine and second signals when the propeller driven by the slave engine trails said predetermined position relative to the propeller driven by the master engine, and means operatively associated with said energizing circuit and connected to receive said first and second signals and operable for increasing the degree of energization of said magnetic coil means 31 resporgse to one of sai signals and to decrease the egree o energization o sai magnetic coil means m response to the other of said signals, the said changes in the degree of energization of said magnetic coil means adjusting the bias on said pilot valve of the slave engine governor in a direction to tend to bring said relative angular positions of said propellers into a desired predetermined relationship.

28. The combination according to claim 27 in which said comparing means comprises a rotating magnet driven by said'master engine and a stationary-coil adjacent to and cooperating with said magnet to emit electrical pulses in unison with the movement of said magnet past said coil, amplifying means to amplify said electrical pulses, said comparing means also comprising a rotatable shaft driven by said slave engine, a pair of fixed photosensitive elements angularly spaced less than about said shaft, a pair of fixed lamps facing said photosensitive elements, means for momentarily illuminating said lamps by the amplified electrical pulses from said amplifying means, a disc located between said lamps and said photosensitive means and mounted for rotation with said shaft, said disc being opaque over a sufficient portion of its surface to block off the light from both lamps to said photosensitive elements when in the null position, said disc exposing one photosensitive element to a light pulse when angularly displaced slightly in one direction from said null position and exposing the other photosensitive element to a light pulse when angularly displaced slightly in the other direction from said null position, each photosensitive element emitting an electrical signal when illuminated, means for amplifying said electrical signals, one said photosensitive elements emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in one direction, the other said photosensitive element emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in the opposite direction, thereby tending to bring the speed of said slave engine into synchronization with said master engine when said opaque portion of said disc is out of the null position at the instant when the light pulses occur. 

1. In combination: a master engine, a slave engine, a variable pitch propeller having pitch adjusting means and driven by said slave engine, a spring loaded governor driven by said slave engine and controlling said pitch adjusting means and operable to cause the propeller pitch to vary directly with variations in speed of said slave engine, adjustable biasing means operatively associated with said governor and energizable for adjusting the load on said governor to adjust the response of said governor to engine speed, means establiShing a speed standard comprising an element actuated by said master engine, comparing means for comparing the speed of said slave engine with said speed standard and operable to develop a signal in response to a difference between said speeds, a source of energy for said biasing means, and control means operable in response to said signal for controlling the supply of energy from said source to said biasing means to adjust the bias on said governor and in a direction to reduce said difference, said comparing means comprises photosensitive means and lamp means for illuminating said photosensitive means and a rotary member between said lamp means and said photosensitive means having one rotated position wherein the light from said lamp means is blocked off from said photosensitive means, said element driven by said master engine being operable for actuating one of said rotary member and lamp means while said slave engine actuates the other thereof, and illumination of said photosensitive means providing said signal for actuating said control means.
 2. The combination according to claim 1 in which said rotary member is a disc having an opaque portion which is disposed between said lamp means and photosensitive means in said one rotated position of said disc, said lamp means when actuated being momentarily energized in a predetermined rotated position of the actuating engine therefor to provide stroboscopic light source means.
 3. The combination according to claim 2 in which said photosensitive means comprises a pair of photosensitive elements each operable to provide a signal for actuation of said control means for adjustment of the supply of energy from said source to said biasing means in a respective direction when illuminated, said disc in said one rotated position thereof blocking off the light from said lamp means to both of said photosensitive elements and when displaced in respective directions from said one rotated position exposing respective ones of said photosensitive elements to the light from said lamp means.
 4. The combination according to claim 3 in which said biasing means is electrically operable and the said source includes a source of electrical energy, said control means comprising first and second control circuits each operable when completed to provide for adjustment of the supply of energy from said source to said biasing means in a respective direction, each photosensitive element being connected in controlling relation to a respective one of said circuits.
 5. The combination according to claim 4 which includes means responsive to the completion of either of said first and second control circuits for preventing the completion of the other thereof.
 6. The combination according to claim 4 in which said biasing means is in the form of a coil operable when energized to establish a magnetic field, said governor including centrifugally operable means comprising at least one magnetic part and movable in response to changes in engine speed, said coil being so mounted as to cause the field thereupon to act on said magnetic part of said governor and in the direction of movement thereof whereby the total load on the governor which controls the governor response to engine speed includes the spring load thereon and the magnetic load thereon, a change in the supply of energy to said coil changing the position of the governor for a given speed of the slave engine.
 7. The combination according to claim 6 in which said governor comprises a reciprocable valve member and said magnetic part is integral therewith.
 8. The combination according to claim 6 in which said governor comprises flyball members, said flyball members being magnetic and forming the magnetic part of said governor acted on by the field of said coil.
 9. The combination according to claim 4 which includes adjustable current controlling means operable for controlling the supply of energy from said source to said biasing means, a reversible motor connected to said current contRolling means for adjustment thereof in opposite directions, said first and second control currents being connected in circuit with said motor and each controlling the energization of said motor in a respective direction.
 10. The combination according to claim 9 in which said current controlling means includes a rheostat, said motor being connected in adjusting relation to said rheostat.
 11. The combination according to claim 1 in which said biasing means is a solenoid coil and said source of energy comprises a source of direct current, a current controlling transistor having the collector-emitter path thereof in series with said coil across said source, biasing resistor means connected across said source and to the base of current controlling transistor, said control means including an adjustable element associated with said resistor means movable for varying the conductivity of said current controlling transistor, a reversible motor connected to said element for adjustment thereof, and energizing circuit means connected to said motor and under the control of said signals from said comparing means.
 12. The combination according to claim 11 in which said energizing circuit means includes first and second transistorized circuits connected to said motor and each effective for causing rotation of the movement of the motor in a respective direction, said first circuit becoming effective in response to a said signal from said comparing means resulting from a leading condition of said slave motor and said second circuit becoming effective in response to a said signal resulting from a trailing condition of said slave motor, each of said first and second circuits having a control terminal to receive the respective signal from said comparing means.
 13. The combination according to claim 12 which includes a transistor for each control terminal having the collector-emitter path thereof connected between ground and the respective control terminal, each transistor having the base thereof connected to the other control terminal whereby a signal supplied to one control terminal will prevent the supply of a signal to the other control terminal.
 14. The combination according to claim 12, in which said resistor means connected to the base of said current controlling means comprises a pair of resistors connected in series across said source and having the juncture thereof connected to the base of said current controlling transistor, a further transistor having the collector-emitter path thereof connected across each of said pair of resistors, and the base of each further resistor being connected to a point in a respective one of said first and second circuits to receive a biasing signal therefrom when the respective circuit is actuated by a signal from said comparing means.
 15. The combination according to claim 13, which includes a second pair of resistors connected in series, switch means selectively operable for interrupting the connection between the bases of each of said further transistors and the respective said first and second circuits while simultaneously connecting said second pairs of resistors across said source, and means connected between the juncture of said coil with the collector-emitter path of said current controlling transistor and the juncture of said second pair of resistors and responsive to the voltage between said junctures to make the one of said first and second circuits effective which will lead to adjustment of said element in a direction to reduce said voltage.
 16. The combination according to claim 15 in which said last mentioned means comprises a pair of transistors each having the emitter connected to a respective one of said junctures and the base connected to the other thereof whereby each of the transistors goes conductive in response to a voltage between said junctures of a respective polarity, the collectors of the said transistors being connected to said first and second circuits to supply actuating signals thereto when the respectivE transistor goes conductive.
 17. The combination according to claim 1 in which said means establishing a speed standard also includes means for establishing a phase standard, said comparing means also comparing the phase of said slave engine with that of said master engine and developing a said signal in conformity with measured differences in phase.
 18. In a twin-engine aircraft having a master engine and a slave engine and a constant speed propeller driven by each engine and each propeller including a fluid motor to adjust the pitch thereof; means for synchronizing the speed of said engines, comprising: a speed sensing fluid governor driven by each said engine, each said governor incorporating a pilot valve regulating the flow of fluid to the respective said fluid motor for maintaining the respective propeller at the proper pitch for a prescribed rotational speed of the engine connected thereto, each said pilot valve being resiliently biased toward pitch reducing position and each governor urging the respective pilot valve toward pitch increasing position as the speed of the pertaining engine increases, said governor driven by said slave engine incorporating unidirectionally acting magnetic means operatively associated with said pilot valve and adapted to develop a unidirectional bias thereon, said magnetic means comprising magnetic coil means and magnet means unidirectionally influenced by the field of said magnetic coil means, one of said magnet means and magnet coil means being connected to the said pilot valve to move therewith and the other thereof being stationarily mounted, an energizing circuit connected to said magnetic coil means to supply energy thereto and including a source of electrical energy, speed comparing means comprising an element actuated by each engine and operable for comparing the rotational speed of said master engine with that of said slave engine, said speed comparing means developing signals which vary in conformity with the relative rotational speed of said engines, and means operatively associated with said energizing circuit and connected to receive said signals and operable for varying the degree of energization of said magnetic coil means by said energizing circuit in response to variations in said signals, variations in the degree of energization of said magnetic coil means adjusting the bias exerted by said magnetic means on said pilot valve of the slave engine governor and in a direction to tend to bring said speed of said slave engine into synchronism with that of said master engine.
 19. The combination according to claim 18 in which said comparing means comprises a rotating magnet driven by said master engine and a stationary coil adjacent to and cooperating with said magnet to emit electrical pulses in unison with the movement of said magnet past said coil, amplifying means to amplify said electrical pulses, said comparing means also comprising a rotatable shaft driven by said slave engine, a pair of fixed photosensitive elements angularly spaced less than 180* about said shaft, a pair of fixed lamps facing said photosensitive elements, means for momentarily illuminating said lamps by the amplified electrical pulses from said amplifying means, a disc located between said lamps and said photosensitive means and mounted for rotation with said shaft, said disc being opaque over a sufficient portion of its surface to block off the light from both lamps to said photosensitive elements when in the null position, said disc exposing one photosensitive element to a light pulse when angularly displaced slightly in one direction from said null position and exposing the other photosensitive element to a light pulse when angularly displaced slightly in the other direction from said null position, each photosensitive element emitting an electrical signal when illuminated, means for amplifying said electrical signals, one said photosensitive elements emitting electrical signals when exposed to light for the purpose of energizing said magnetIc coil means in a manner to bias said pilot valve in one direction, the other said photosensitive element emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in the opposite direction, thereby tending to bring the speed of said slave engine into synchronization with said master engine when said opaque portion of said disc is out of the null position at the instant when the light pulses occur.
 20. The combination according to claim 19 wherein said rotating magnet is driven by said slave engine and said rotating shaft is driven by said master engine.
 21. The combination according to claim 19, including means for adjusting the angular position of said photosensitive elements, together with the corresponding lamps, about said rotatable shaft.
 22. The combination according to claim 19, including means for adjusting the angular position of said coil about the shaft of said rotating magnet.
 23. The combination according to claim 19, including means for automatically controlling the current supplied to said magnetic coils according to a prescribed schedule when said disc is not in the null position.
 24. In combination according to claim 19, including means for energizing said magnetic coils with a constant current in one direction to provide a reference bias on said pilot valve, said means including means for automatically changing the degree of bias when said disc is displaced from the null position a predetermined amount.
 25. In combination according to claim 24 in which said means for automatically changing the degree of bias includes a motor driven potentiometer for the purpose of changing the current supplied to said magnetic coil.
 26. In combination according to claim 19 including means for blocking out the signal emitted by the second photosensitive element which becomes uncovered by the opaque portion of said disc after the said opaque portion becomes displaced from said null position, in order that the signal from the first photosensitive element which becomes uncovered can act without opposition from said second photosensitive element to energize said coil in a direction to correct the out of synchronous condition when the rotational speed of said slave engine is above or below the speed of said master engine by a prescribed amount.
 27. In a twin-engine aircraft having a master engine and a slave engine and a constant speed propeller driven by each engine and each propeller including a fluid motor to adjust the pitch thereof; means for synchronizing the speed of said engines, comprising: a speed sensing fluid governor driven by each said engine, each said governor incorporating a pilot valve regulating the flow of fluid to the respective said fluid motor for maintaining the respective propeller at the proper pitch for a prescribed rotational speed of the engine connected thereto, each said pilot valve being resiliently biased toward pitch reducing position and each governor urging the respective pilot valve toward pitch increasing position as the speed of the pertaining engine increases, said governor driven by said slave engine incorporating unidirectionally acting magnetic means operatively associated with said pilot valve and adapted to develop a unidirectional bias thereon, said magnetic means comprising magnetic coil means and magnet means unidirectionally influenced by the field of said magnetic coil means, one of said magnet means and magnet coil means being connected to the said pilot valve to move therewith and the other thereof being stationarily mounted, an energizing circuit connected to said magnetic coil means to supply energy thereto and including a source of electrical energy, comparing means comprising an element actuated by each engine and operable for comparing the angular position of the propeller driven by said slave engine with the angular position of the propeller driven by said master engine, said comparing means developing first signaLs when said propeller driven by the slave engine leads a predetermined position relative to the propeller driven by the master engine and second signals when the propeller driven by the slave engine trails said predetermined position relative to the propeller driven by the master engine, and means operatively associated with said energizing circuit and connected to receive said first and second signals and operable for increasing the degree of energization of said magnetic coil means in response to one of said signals and to decrease the degree of energization of said magnetic coil means in response to the other of said signals, the said changes in the degree of energization of said magnetic coil means adjusting the bias on said pilot valve of the slave engine governor in a direction to tend to bring said relative angular positions of said propellers into a desired predetermined relationship.
 28. The combination according to claim 27 in which said comparing means comprises a rotating magnet driven by said master engine and a stationary coil adjacent to and cooperating with said magnet to emit electrical pulses in unison with the movement of said magnet past said coil, amplifying means to amplify said electrical pulses, said comparing means also comprising a rotatable shaft driven by said slave engine, a pair of fixed photosensitive elements angularly spaced less than 180* about said shaft, a pair of fixed lamps facing said photosensitive elements, means for momentarily illuminating said lamps by the amplified electrical pulses from said amplifying means, a disc located between said lamps and said photosensitive means and mounted for rotation with said shaft, said disc being opaque over a sufficient portion of its surface to block off the light from both lamps to said photosensitive elements when in the null position, said disc exposing one photosensitive element to a light pulse when angularly displaced slightly in one direction from said null position and exposing the other photosensitive element to a light pulse when angularly displaced slightly in the other direction from said null position, each photosensitive element emitting an electrical signal when illuminated, means for amplifying said electrical signals, one said photosensitive elements emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in one direction, the other said photosensitive element emitting electrical signals when exposed to light for the purpose of energizing said magnetic coil means in a manner to bias said pilot valve in the opposite direction, thereby tending to bring the speed of said slave engine into synchronization with said master engine when said opaque portion of said disc is out of the null position at the instant when the light pulses occur. 